MODE AND CONDITIONS OF GROWTH 443 



§ 8. External Conditions and Growth 



The normal mode and rate of growth are subject to 

 modification by the conditions of the environment. The 

 response of the plant to their impact is extremely complex, 

 for, as we have said, growth sums up the whole metabolic 

 activity of the plant, and the external conditions may affect 

 any or all of the links in the chain of processes. We may take 

 as an example the case of light. It acts in the first place 

 through photosynthesis, and if it limits that process it limits 

 also the supply of organic building material. It also acts 

 directly on the process of extension, and in a very complex' 

 way. A plant grown in the dark is etiolated, it possesses no 

 chlorophyll, it is drawn, and abnormally elongated, the leaves 

 do not expand. Yet it has been shown by Vogt (191 5) that 

 if the coleoptile of an oat, grown in the dark, is illuminated 

 for a few seconds, after a transitory sUght decrease in 

 growth rate, there follows a much more marked though 

 also transitory increase. Sierp (1917, 1918) found that if the 

 illumination was continued, the increased growth rate was 

 maintained. This apparently contradictory result — that 

 light increases growth-rate — is explained by the fact that 

 though the rate of growth is higher in the light than in the 

 dark, the grand period is hastened on and passed through at a 

 lower rate than in the dark : the illuminated seedling actually 

 grows faster than the darkened, but it does not attain the same 

 length. We do not know exactly how illumination affects 

 growth, but it may be through induced changes in the 

 permeability of the cells. The phenomena of etiolation 

 have other causes than the direct effect of light on growth- 

 rate ; the formation of chlorophyll and other chemical 

 changes may be important, and it has been shown by Priestley 

 and Ewing (1923) that changes in the endoderm occur. 

 Sufficient has been said to show the complex action of this 

 single factor. 



The influence of temperature on the growth of the pea 

 radicle has already been described ; temperature, too, has 



